Hi Judita,
Bus 350 does offer service from CDG Airport Roissypole/Terminal 3 station to Paris Gare de l'Est (just east of Gare du Nord). For information on getting to Roissypole Terminal 3, see my
Charles de Gaulle Terminal 1 to Paris photo tour article which has instructions on getting to the station.
But, the price is
supposed to be the same price of a bus trip from Zone 5 to Zone 1, which is 5
ticket t+, each costing €1.60 which means 8€.
The reason why some people say it's the cheapest is that getting on the bus requires only one ticket. Whether one is honest enough to give the bus driver the other four tickets is... at their discretion. Plus, I don't see the bus driver stopping the bus to find those who didn't pay enough tickets and kick them off half way through the roughly 1 hour trip. His primary job is to drive the bus, and secondarily act as "controller". Although the driver may question passengers as to how far they're going when he sees a bunch of luggage in tow and asks the rider to validate the correct number of tickets.
Overall, I recommend the
RER B to go from Charles de Gaulle Airport to Paris. Mostly for its speed and convenience.
Enjoy!
Ben